Machines used to copy, print, or otherwise process sheet documents must include means for properly aligning the individual documents before they are processed. Sheet documents may be skewed out of proper alignment when initially loaded into the processing machine by an operator, or the documents may become skewed during their passage through the machine's document flow path. Therefore, sheet-processing machines usually provide some form of "skew control" means to assure proper alignment when the document is fed into the machine's processing station.
[NOTE: As used herein, the term "document" refers generally to a single sheet of paper, plastic laminate, or similar sheet material used to record information.]
In sheet processing machines known to us, the most common method of achieving proper sheet alignment is "edge" registration wherein drive rollers direct one side of the document against a registration wall or flange (e.g., see U.S. Pat. No. 4,483,530 issued to Spencer et al. and U.S. Pat. No. 4,877,234 issued to Mandel). However, other known sheet processing machines achieve proper sheet alignment by using a "center" registration method wherein skew is recognized by sensing the leading edge of the document which is then moved until it is properly aligned in the exact center of the document flow path. Examples of the latter method can be found in U.S. Pat. No. 3,952,866 (Leloux), in which the document is captured and centered by a plurality of needles, and also in U.S. Patent No. 5,219,159 (Malachowski et al.) in which the document is secured by stalled rollers that are then moved transverse to the document flow path to achieve appropriate centering. Such known registration systems are mechanically complex and relatively expensive to manufacture, assemble, and service.
Machines used to copy engineering-size documents (as different from conventional letter-size documents) have document feeding and transport mechanisms that require the operator to manually insert the document to be copied. While such manual handling is time consuming and expensive, it has been accepted as necessary because drawings and other engineering documents are recorded on a variety of materials and come in such large and varied sizes. For example, the sheet materials often vary in thickness, while also varying in widths ranging from 8 inches (20 cm) to 3 feet (90 cm), and varying even more in length, with some drawings coming in large rolls having lengths measuring several yards (meters).
These large variations in engineering document parameters make document loading, reliable feeding, and separation of documents very difficult; and deskewing is particularly difficult because the beam strength of thin sheet material is significantly reduced as the width of the sheet increases. Therefore, proper registration and skew control is particularly important in machines used for processing these variably-sized engineering documents. All of these problems have heretofore prevented the development of commercially-acceptable automatic document feeder ("ADF") apparatus for processing engineering-type documents.